The present invention relates generally to amplifier circuits, and in more particular, to a variable gain current amplifier circuit that includes a differential pair in a feedback loop.
Variable gain amplifiers are used in many systems. For example, communication systems such as wireless phones and ultrasound imaging systems are two applications that rely on variable gain amplification to improve performance. Variable gain amplification allows an output signal of the amplifier to use the full range of the amplitude, which minimizes the effects of noise and distortion. An example of this is shown in FIG. 1. As can be seen, the top input signal needs to be attenuated, while the bottom input signal needs to be amplified.
Variable gain amplifiers may be based on voltage, current or charge. Voltage mode amplifiers are probably the most widely used. Examples of such include complex circuits where the amplification is provided by discrete transconductance stages. However, due to the requirements of modern digital circuits, integrated circuit fabrication processes have resulted in semiconductor devices with lower breakdown voltages. This has enabled supply of these integrated circuits to be continually lowered. Although, this development has adversely effected voltage mode amplifiers since the dynamic range of such circuits have been reduced.
Charge mode amplifiers are one alternative. However, such a circuit utilizes a discrete time technique that is not suitable for high-speed operation. In contrast, current mode amplifiers are less constrained by reduced power supplies and are able to operate at very high speeds. However, these circuits currently suffer from other drawbacks.
For example, a differential pair circuit may be used as a current mode variable gain amplifier. However, this circuit is only capable of attenuating signals and not amplifying. A fixed gain amplifier could be placed in front of a differential pair in order to provide a wider range of gains. However, this approach would waste power.
Another type of current mode variable gain amplifier relies on the exponential relationship between the base-emitter voltage and collector current of a bipolar transistor. However, this circuit is to sensitive to the emitter resistance of the bipolar transistor, which could cause gain errors and distortion.
The present invention is directed to a current amplifier. The current amplifier according to the present invention includes a voltage amplifier with a predetermined gain, an input and an output. A transistor is connected to the output of the voltage amplifier. A differential pair of transistors is connected between the transistor and the input of the voltage amplifier. The gain of the current amplifier varies according to the to the voltage applied to the input of the differential pair of transistors. Further, a current through the transistor will be divided into a feedback current and an output current according to the voltage applied to the input of the differential pair of transistors.
The present invention is also directed to a method of amplifying a current. The method according to the present invention includes a voltage being developed that is proportional to the current. The voltage is amplified according to a predetermined gain to produce an amplified voltage. A second current is produced that is proportional to the amplified voltage. Further, the second current is divided into a feedback current and an output current according to a control voltage.